Fix misleading comments. That function only determines whether replacements cost more; it doesn't actually *validate* costs as being cheaper. For example, it returns true also if it for various reasons cannot determine the costs, or if the new cost is the same, like when doing an identity replacement. The code has been the same since r0-59417-g64b8935d4809f3. * combine.cc: Correct comments about combine_validate_cost.